It has been suggested that an improved conductivity lateral insulated gate transistor can be achieved by the addition of a split or dual conductivity anode to establish a lateral diode current path between the cathode and anode. FIGS. 1 and 2 of this application show alternate forms of these devices. Heretofore efforts to establish such a lateral diode current have been unsuccessful inasmuch as the lateral current flow has been established only at the expense of device performance.
FIG. 1 is an example of one such previously proposed dual anode insulated gate transistor wherein the conventional anode has been replaced with an anode comprising alternate conductivity type emitter regions illustrated as P and N type conductivity regions. It has been suggested that a lateral diode current can be established from the P+ base region to the N+ anode region. A metal layer is disposed over and in ohmic electrical contact with both N+ and P+ emitter regions to function as an anode electrode and also to short the emitter regions to each other to prevent diode action between them. This particular structure, however, has been found inadequate to provide the proposed lateral diode action without also degrading the operation of the insulated gate transistor. More particularly, the N+ portion of the anode acts as a collector and collects a sufficient amount of the lateral electron current to short out the effectiveness of the P+ portion of the anode and thus suppress the injection of minority carriers from the P+ anode into the N drift layer during the on-state operation of the device. Consequently, this previously proposed device cannot provide for low resistance reverse conduction without impairing the enhanced conductivity characteristic of a lateral insulated gate transistor which is attributable to the bipolar conduction achieved by minority carrier injection. The previously proposed structure degrades the drift layer conductivity modulation essential to lateral insulated gate transistor performance.
Another proposed design has been disclosed in the article "Analysis of the Lateral Insulated Gate Transistor" by M. R. Simpson, P. A. Gough, F. I. Hshieh and V. Rumennik published in the Technical Digest of the International Electron Device Meeting, 1985, pages 740-743. An illustration of the first figure of this article is shown as FIG. 2 of this application. In particular, this article proposes to provide a substrate having a thickness of approximately 300 microns and an electrical terminal for grounding. A thin drift layer is provided atop the substrate. A large and heavily doped deep P+ region adjacent a smaller and shallower heavily doped N+ region is provided within the drift layer. This article discloses that the current voltage characteristic of the proposed device, has a pronounced on-state knee at approximately 1.2 volts. After the voltage is increased beyond 1.2 volts, the current gradually increases. Thus, the operating characteristic of this proposed device is in marked contrast to the operating characteristic of a conventional lateral insulated gate transistor in which the on- state knee occurs at 0.7 volts and thereafter the current rapidly increases. The higher on-state knee voltage of the Simpson et al. design is caused by the electron current bypassing the anode junction between the P+ region and the N drift layer. The electron current flows under the P+ anode region to the N+ anode region. In this prior device the leading edge of the P+ region has been placed in close proximity to the N+ region. The pinch resistance R.sub.p in the drift region beneath the anode is small, and consequently a large electron current must flow before the junction becomes forward biased. Simpson et al. have reported that when the lateral voltage drop exceeds 1.2 volts between the cathode and the anode, the PN junction between the P+ anode and the N type drift layer will become sufficiently forward biased to inject minority carriers into the drift layer to establish bipolar conduction in the lateral insulated gate transistor device. Before such proposed device achieves the 1.2 volt drop, however, the P-N junction between the P+ anode and the drift layer does not inject enough minority carriers to modulate the conductivity of the drift layer. Therefore, this prior device is current controlled inasmuch as it requires a large level of current flow before minority carrier junction is established.
There thus exists an unfilled need to provide a lateral insulated gate transistor structure containing a reverse conducting diode integral therein without degrading of the forward conduction characteristic of the lateral insulated gate transistor.